Chapter 21 The STL (maps and algorithms) Bjarne Stroustrup www. stroustrup. com/Programming
Overview n n Common tasks and ideals Containers, algorithms, and iterators The simplest algorithm: find() Parameterization of algorithms n n Sequence containers n n n map, set Standard algorithms n n n vector and list Algorithms and parameterization revisited Associative containers n n find_if() and function objects copy, sort, … Input iterators and output iterators List of useful facilities n Headers, algorithms, containers, function objects Stroustrup/Programming Nov'13 3
Basic model n A pair of iterators defines a sequence n The beginning (points to the first element – if any) n The end (points to the one-beyond-the-last element) begin: end: … § An iterator is a type that supports the “iterator operations” of § ++ Point to the next element § * Get the element value § == Does this iterator point to the same element as that iterator? § Some iterators support more operations (e. g. , --, +, and [ ]) Stroustrup/Programming Nov'13 4
Accumulate (sum the elements of a sequence) template<class In, class T> T accumulate(In first, In last, T init) { while (first!=last) { init = init + *first; ++first; } return init; } v: int sum = accumulate(v. begin(), v. end(), 0); 1 2 3 4 // sum becomes 10 Stroustrup/Programming Nov'13 5
Accumulate (sum the elements of a sequence) void f(vector<double>& vd, int* p, int n) { double sum = accumulate(vd. begin(), vd. end(), 0. 0); // add the elements of vd // note: the type of the 3 rd argument, the initializer, determines the precision used int si = accumulate(p, p+n, 0); // sum the ints in an int (danger of overflow) // p+n means (roughly) &p[n] long sl = accumulate(p, p+n, long(0)); // sum the ints in a long double s 2 = accumulate(p, p+n, 0. 0); // sum the ints in a double // popular idiom, use the variable you want the result in as the initializer: double ss = 0; ss = accumulate(vd. begin(), vd. end(), ss); // do remember the assignment } Stroustrup/Programming Nov'13 6
Accumulate (generalize: process the elements of a sequence) // we don’t need to use only +, we can use any binary operation (e. g. , *) // any function that “updates the init value” can be used: template<class In, class T, class Bin. Op> T accumulate(In first, In last, T init, Bin. Op op) { while (first!=last) { init = op(init, *first); // means “init op *first” ++first; } return init; } Stroustrup/Programming Nov'13 7
Accumulate // often, we need multiplication rather than addition: #include <numeric> Note: multiplies for * #include <functional> void f(list<double>& ld) { double product = accumulate(ld. begin(), ld. end(), 1. 0, multiplies<double>()); // … } Note: initializer 1. 0 // multiplies is a standard library function object for multiplying Stroustrup/Programming Nov'13 8
Accumulate (what if the data is part of a record? ) struct Record { int units; double unit_price; // … }; // number of units sold // let the “update the init value” function extract data from a Record element: double price(double v, const Record& r) { return v + r. unit_price * r. units; } void f(const vector<Record>& vr) { double total = accumulate(vr. begin(), vr. end(), 0. 0, price); // … } Stroustrup/Programming Nov'13 9
Accumulate (what if the data is part of a record? ) struct Record { int units; double unit_price; // … }; // number of units sold void f(const vector<Record>& vr) { double total = accumulate(vr. begin(), vr. end(), 0. 0, price(double v, const Record& r) { return v + r. unit_price * r. units; } ); // … } // Is this clearer or less clear than the price() function? Stroustrup/Programming Nov'13 10
Inner product template<class In, class In 2, class T> T inner_product(In first, In last, In 2 first 2, T init) // This is the way we multiply two vectors (yielding a scalar) { while(first!=last) { init = init + (*first) * (*first 2); // multiply pairs of elements and sum ++first; ++first 2; } number of units 1 2 3 4 … return init; * * * } unit price 4 3 2 1 … Stroustrup/Programming Nov'13 11
Inner product example // calculate the Dow-Jones industrial index: vector<double> dow_price; // share price for each company dow_price. push_back(81. 86); dow_price. push_back(34. 69); dow_price. push_back(54. 45); // … vector<double> dow_weight; // weight in index for each company dow_weight. push_back(5. 8549); dow_weight. push_back(2. 4808); dow_weight. push_back(3. 8940); // … double dj_index = inner_product( // multiply (price, weight) pairs and add dow_price. begin(), dow_price. end(), dow_weight. begin(), 0. 0); Stroustrup/Programming Nov'13 12
Inner product example // calculate the Dow-Jones industrial index: vector<double> dow_price = { // share price for each company 81. 86, 34. 69, 54. 45, // … }; vector<double> dow_weight = { // weight in index for each company 5. 8549, 2. 4808, 3. 8940, // … }; double dj_index = inner_product( // multiply (price, weight) pairs and add dow_price. begin(), dow_price. end(), dow_weight. begin(), 0. 0); Stroustrup/Programming Nov'13 13
Inner product (generalize!) // we can supply our own operations for combining element values with“init”: template<class In, class In 2, class T, class Bin. Op 2 > T inner_product(In first, In last, In 2 first 2, T init, Bin. Op op, Bin. Op 2 op 2) { while(first!=last) { init = op(init, op 2(*first, *first 2)); ++first 2; } return init; } Stroustrup/Programming Nov'13 14
Map (an associative array) n n For a vector, you subscript using an integer For a map, you can define the subscript to be (just about) any type Key type int main() { map<string, int> words; for (string s; cin>>s; ) ++words[s]; Value type // keep (word, frequency) pairs // note: words is subscripted by a string // words[s] returns an int& // the int values are initialized to 0 for (const auto& p : words) cout << p. first << ": " << p. second << "n"; } Stroustrup/Programming Nov'13 15
An input for the words program (the abstract) This lecture and the next presents the STL (the containers and algorithms part of the C++ standard library). It is an extensible framework dealing with data in a C++ program. First, I present the general ideal, then the fundamental concepts, and finally examples of containers and algorithms. The key notions of sequence and iterator used to tie containers (data) together with algorithms (processing) are presented. Function objects are used to parameterize algorithms with “policies”. Stroustrup/Programming Nov'10 16
(data): 1 (processing): 1 (the: 1 C++: 2 First, : 1 Function: 1 It: 1 STL: 1 The: 1 This: 1 algorithms: 3 algorithms. : 1 and: 5 are: 2 concepts, : 1 containers: 3 data: 1 dealing: 1 examples: 1 extensible: 1 finally: 1 framework: 1 fundamental: 1 general: 1 ideal, : 1 in: 1 is: 1 Output (word frequencies) iterator: 1 key: 1 lecture: 1 library). : 1 next: 1 notions: 1 objects: 1 of: 3 parameterize: 1 part: 1 presented. : 1 presents: 1 program. : 1 sequence: 1 standard: 1 the: 5 then: 1 tie: 1 to: 2 together: 1 used: 2 with: 3 “policies”. : 1 Stroustrup/Programming Nov'13 17
Map (an associative array) n n For a vector, you subscript using an integer For a map, you can define the subscript to be (just about) any type Key type int main() { map<string, int> words; for (string s; cin>>s; ) ++words[s]; Value type // keep (word, frequency) pairs // note: words is subscripted by a string // words[s] returns an int& // the int values are initialized to 0 for (const auto& p : words) cout << p. first << ": " << p. second << "n"; } Stroustrup/Programming Nov'13 18
Map n After vector, map is the most useful standard library container n n Maps (and/or hash tables) are the backbone of scripting languages A map is really an ordered balanced binary tree n n By default ordered by < (less than) For example, map<string, int> fruits; Map node: fruits: Orange 99 Grape 100 Apple 7 Kiwi 2345 Key first Value second Node* left Node* right … Quince 0 Plum 8 Stroustrup/Programming Nov'13 19
Map Some implementation defined type // note the similarity to vector and list template<class Key, class Value> class map { // … using value_type = pair<Key, Value>; // a map deals in (Key, Value) pairs using Iterator = ? ? ? ; using const_iterator = ? ? ? ; iterator begin(); iterator end(); // probably a pointer to a tree node // points to first element // points to one beyond the last element Value& operator[ ](const Key&); // get Value for Key; creates pair if // necessary, using Value( ) iterator find(const Key& k); // is there an entry for k? void erase(iterator p); // remove element pointed to by p pair<iterator, bool> insert(const value_type&); // insert new (Key, Value) pair // … // the bool is false if insert failed }; Stroustrup/Programming Nov'13 20
Map example (build some maps) map<string, double> dow; // Dow-Jones industrial index (symbol, price) , 03/31/2004 // http: //www. djindexes. com/jsp/industrial. Averages. jsp? side. Menu=true. html dow["MMM"] = 81. 86; dow["AA"] = 34. 69; dow["MO"] = 54. 45; // … map<string, double> dow_weight; // dow (symbol, weight) dow_weight. insert(make_pair("MMM", 5. 8549)); // just to show that a Map // really does hold pairs dow_weight. insert(make_pair("AA", 2. 4808)); dow_weight. insert(make_pair("MO", 3. 8940)); // and to show that notation matters // … map<string, string> dow_name; // dow (symbol, name) dow_name["MMM"] = "3 M Co. "; dow_name["AA"] = "Alcoa Inc. "; dow_name["MO"] = "Altria Group Inc. "; // … Stroustrup/Programming Nov'13 21
![Map example (some uses) double alcoa_price = dow["AA"]; double boeing_price = dow["BO"]; // read](https://present5.com/presentation/00e1bdeb08b12be92d348900eeb2ff9b/image-21.jpg "Map example (some uses) double alcoa_price = dow[\"AA\"]; double boeing_price = dow[\"BO\"]; // read")
Map example (some uses) double alcoa_price = dow["AA"]; double boeing_price = dow["BO"]; // read values from a map if (dow. find("INTC") != dow. end()) cout << "Intel is in the Down"; // look in a map for an entry // iterate through a map: for (const auto& p : dow) { const string& symbol = p. first; // the "ticker" symbol cout << symbol << 't' << p. second << 't' << dow_name[symbol] << 'n'; } Stroustrup/Programming Nov'13 22
Map example (calculate the DJ index) double value_product( const pair<string, double>& a, const pair<string, double>& b) { return a. second * b. second; } // extract values and multiply double dj_index = inner_product(dow. begin(), dow. end(), dow_weight. begin(), 0. 0, plus<double>(), value_product ); Stroustrup/Programming Nov'13 // all companies in index // their weights // initial value // add (as usual) // extract values and weights // and multiply; then sum 23
Containers and “almost containers” n Sequence containers n n Associative containers n n array, string, stack, queue, priority_queue, bitset New C++11 standard containers n n map, set, multimap, multiset “almost containers” n n vector, list, deque unordered_map (a hash table), unordered_set, … For anything non-trivial, consult documentation n Online n n SGI, Rogue. Wave, Dinkumware Other books n n n Stroustrup: The C++ Programming language 4 th ed. (Chapters 30 -33, 40. 6) Austern: Generic Programming and the STL Josuttis: The C++ Standard Library Stroustrup/Programming Nov'13 24
Algorithms n An STL-style algorithm n Takes one or more sequences n n Takes one or more operations n n n Usually as pairs of iterators Usually as function objects Ordinary functions also work Usually reports “failure” by returning the end of a sequence Stroustrup/Programming Nov'13 25
Some useful standard algorithms n r=find(b, e, v) r=find_if(b, e, p) x=count(b, e, v) x=count_if(b, e, p) sort(b, e, p) copy(b, e, b 2) n unique_copy(b, e, b 2) n merge(b, e, b 2, e 2, r) n r=equal_range(b, e, v) n equal(b, e, b 2) n n n r points to the first occurrence of v in [b, e) r points to the first element x in [b, e) for which p(x) x is the number of occurrences of v in [b, e) x is the number of elements in [b, e) for which p(x) sort [b, e) using < sort [b, e) using p copy [b, e) to [b 2, b 2+(e-b)) there had better be enough space after b 2 copy [b, e) to [b 2, b 2+(e-b)) but don’t copy adjacent duplicates merge two sorted sequence [b 2, e 2) and [b, e) into [r, r+(e-b)+(e 2 -b 2)) r is the subsequence of [b, e) with the value v (basically a binary search for v) do all elements of [b, e) and [b 2, b 2+(e-b)) compare equal? Stroustrup/Programming Nov'13 26
Copy example template<class In, class Out> Out copy(In first, In last, Out res) { while (first!=last) *res++ = *first++; // conventional shorthand for: // *res = *first; ++res; ++first return res; } void f(vector<double>& vd, list<int>& li) { if (vd. size() < li. size()) error("target container too small"); copy(li. begin(), li. end(), vd. begin()); // note: different container types // and different element types // (vd better have enough elements // to hold copies of li’s elements) sort(vd. begin(), vd. end()); // … } Stroustrup/Programming Nov'13 27
Input and output iterators // we can provide iterators for output streams ostream_iterator<string> oo(cout); *oo = "Hello, "; ++oo; *oo = "world!n"; // assigning to *oo is to write to cout // meaning cout << "Hello, " // “get ready for next output operation” // meaning cout << "world!n" // we can provide iterators for input streams: istream_iterator<string> ii(cin); string s 1 = *ii; ++ii; string s 2 = *ii; // reading *ii is to read a string from cin // meaning cin>>s 1 // “get ready for the next input operation” // meaning cin>>s 2 Stroustrup/Programming Nov'13 28
Make a quick dictionary (using a vector) int main() { string from, to; cin >> from >> to; // get source and target file names ifstream is(from); ofstream os(to); // open input stream // open output stream istream_iterator<string> ii(is); istream_iterator<string> eos; ostream_iterator<string> oo(os, "n"); // make input iterator for stream // input sentinel (defaults to EOF) // make output iterator for stream // append "n" each time // b is a vector initialized from input // sort the buffer // copy buffer to output, // discard replicated values vector<string> b(ii, eos); sort(b. begin(), b. end()); unique_copy(b. begin(), b. end(), oo); } Stroustrup/Programming Nov'13 29
An input file (the abstract) This lecture and the next presents the STL (the containers and algorithms part of the C++ standard library). It is an extensible framework dealing with data in a C++ program. First, I present the general ideal, then the fundamental concepts, and finally examples of containers and algorithms. The key notions of sequence and iterator used to tie containers (data) together with algorithms (processing) are presented. Function objects are used to parameterize algorithms with “policies”. Stroustrup/Programming Nov'13 30
(data) (processing) (the C++ First, Function I It STL The This a algorithms. an and are concepts, containers data dealing examples extensible finally Framework fundamental general ideal, Part of the output in is iterator key lecture library). next notions objects of parameterize part presented. presents program. sequence standard then tie to together used with Stroustrup/Programming Nov'13 “policies”. 31
Make a quick dictionary (using a vector) n We are doing a lot of work that we don’t really need n n Why store all the duplicates? (in the vector) Why sort? Why suppress all the duplicates on output? Why not just n n Put each word in the right place in a dictionary as we read it? In other words: use a set Stroustrup/Programming Nov'13 32
Make a quick dictionary (using a set) int main() { string from, to; cin >> from >> to; // get source and target file names ifstream is(from); ofstream os(to); // make input stream // make output stream istream_iterator<string> ii(is); istream_iterator<string> eos; ostream_iterator<string> oo(os, "n"); // make input iterator for stream // input sentinel (defaults to EOF) // make output iterator for stream // append "n" each time // b is a set initialized from input // copy buffer to output set<string> b(ii, eos); copy(b. begin(), b. end(), oo); } // simple definition: a set is a map with no values, just keys Stroustrup/Programming Nov'13 33
Set n A set is really an ordered balanced binary tree n n By default ordered by < For example, set<string> fruits; set node: Key first fruits: Grape Apple Kiwi Node* left Node* right … Orange Quince Plum Stroustrup/Programming Nov'13 34
copy_if() // a very useful algorithm (missing from the standard library): template<class In, class Out, class Pred> Out copy_if(In first, In last, Out res, Pred p) // copy elements that fulfill the predicate { while (first!=last) { if (p(*first)) *res++ = *first; ++first; } return res; } Stroustrup/Programming Nov'13 35
copy_if() void f(const vector<int>& v) // “typical use” of predicate with data // copy all elements with a value less than 6 { vector<int> v 2(v. size()); copy_if(v. begin(), v. end(), v 2. begin(), [](int x) { return x<6; } ); // … } Stroustrup/Programming Nov'13 36
Some standard function objects n From <functional> n Binary n n n Unary n n n plus, minus, multiplies, divides, modulus equal_to, not_equal_to, greater, less, greater_equal, less_equal, logical_and, logical_or negate logical_not Unary (missing, write them yourself) n less_than, greater_than, less_than_or_equal, greater_than_or_equal Stroustrup/Programming Nov'13 37
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